[TowerTalk] Power Distribution

[Jim Brown]

On Mon, 02 Aug 2010 11:08:25 -0400, Joe Subich, W4TV wrote:

> >> ##  If the business's  don't get a neutral... then how do the
> >> business's obtain 120 vac ??

>Their supply is either 120/208 (e.g. wye or star) with neutral
>being the center of the wye or they need to install their own
>480 to 240/120 transformer on premesis.

That's ordinary 3-phase power. 

The High Leg Delta distribution that I described is 240V Delta. A 
pure delta configuration has no neutral. The High Leg Delta is 
different -- ONE side of the delta is center-tapped to provide a 
neutral. A single phase user gets THAT center-tapped leg. A 3-
phase user gets all three phases (and perhaps the neutral). There 
is 208V between neutral and the leg opposite the center tap. 
That's the High Leg. Also called Wild Leg.

High Leg Delta is VERY widely used in cities and towns where there 
are small businesses and residences on the same street, because 
the same simple system can be used to feed all of them. It's also 
what's I have up here in the Santa Cruz Mountains, fairly far off 
the beaten track. 

Think about WHY there is 3-phase power -- it's because nearly all 
mains power comes from 3-phase generators, AND because big motors 
want to run on 3-phase power. For other uses, single phase power 
is just as efficient. AND single phase power doesn't have the 
neutral harmonic current problem. 

When most loads connected to the power system drew sinusoidal 
current, designers could minimize neutral current by balancing 
loads between the three phases. That hasn't been true for at least 
40 years -- remember the Towering Inferno?  There was a real one, 
and the root cause was that the neutral current was nearly double 
the current on a phase! That's what happens when nearly all loads 
are rich in harmonics. That includes virtually anything with a 
power supply, and all lighting except for old fashioned 
incandescent light bulbs. Nowadays, even big motors run on pulsed 
3-phase power, so the power supplies that create those pulses are 
all harmonics! 

73, Jim K9YC